The invention relates to a process for the production of monocrystalline Hg.sub.1-x Cd.sub.x Te, wherein a molten mass of the desired composition is cooled and solidified in a closed vessel, and the solidified crystal is converted, at temperatures below its melting point, by high temperature recrystallization into the monocrystalline state.
Preferably the pseudobinary compound Hg.sub.1-x Cd.sub.x Te is used as material for highly sensitive infrared detectors, and, in particular, preferably for the wavelength range of the so-called second and third atmospheric windows (3-5 .mu.m wavelength, x=0.3, and 8-12 .mu.m wavelength, x=0.2, respectively; x=mole fraction of CdTe), both photoconducting detectors (photoresistors) and photovoltaic detectors (photodiodes) are used.
The compound Hg.sub.1-x Cd.sub.x Te exhibits a continuous miscibility between the semimetal HgTe (x=0) and the semiconductor compound CdTe (x=1) so that by selection of x, a band gap between 0.3 eV and 1.6 eV can be set. The relevant composition for the infrared spectral region lies in the order of magnitude of 0.15&lt;x&lt;1.0.
For high resolution infrared detectors, a monocrystalline material with as few crystalline defects as possible is essential. Crystal defects are, for example, dislocations, accumulations of dislocations, small angle grain boundaries and common grain boundaries. Crystal defects decrease the lifetime of the minority carriers by formation of recombination centres and cause additional noise. Both effects result in a decrease in the spectral detectivity, which provides an indication of the resolution of the infrared detector.
In the manufacture of Hg.sub.1-x Cd.sub.x Te be volume crystal growth, it is preferable to use crystal growing processes wherein the homogeneous molten mass of the desired composition is directionally cooled at relatively high crystallization speeds in order to suppress a segregation of the molten mass during the rapid cooling process. The synthesis and subsequent rapid cooling process are generally carried out in closed cylindrical quartz glass ampules since the relatively high mercury partial pressure in the molten mass in an open system would result in an inadmissible loss of mercury.
The crystal existing after the cooling is polycrystalline with areas rich in CdTe and HgTe within the grains (so-called dendrites) and a high dislocation density. The conversion of the crystal into the necessary monocrystalline state generally takes place by way of high temperature recrystallization in the solid state at temperatures slightly below the solidus temperature so as to avoid melting of the crystal and consequent material segregation. With this process, the composition differences within the grains are balanced by interdiffusion instead of by simultaneous grain boundary growth of individual grains. As a rule, the recrystallization is performed in the cylindrical ampule used for the synthesis with temperature constancy prevailing between the crystal tip and the crystal end.